Layouts and Rendering in Rails¶ ↑
This guide covers the basic layout features of Action Controller and Action View.
After reading this guide, you will know:
-
How to use the various rendering methods built into Rails.
-
How to create layouts with multiple content sections.
-
How to use partials to DRY up your views.
-
How to use nested layouts (sub-templates).
Overview: How the Pieces Fit Together¶ ↑
This guide focuses on the interaction between Controller and View in the Model-View-Controller triangle. As you know, the Controller is responsible for orchestrating the whole process of handling a request in Rails, though it normally hands off any heavy code to the Model. But then, when it's time to send a response back to the user, the Controller hands things off to the View. It's that handoff that is the subject of this guide.
In broad strokes, this involves deciding what should be sent as the response and calling an appropriate method to create that response. If the response is a full-blown view, Rails also does some extra work to wrap the view in a layout and possibly to pull in partial views. You'll see all of those paths later in this guide.
Creating Responses¶ ↑
From the controller's point of view, there are three ways to create an HTTP response:
-
Call
render
to create a full response to send back to the browser -
Call
redirect_to
to send an HTTP redirect status code to the browser -
Call
head
to create a response consisting solely of HTTP headers to send back to the browser
Rendering by Default: Convention Over Configuration in Action¶ ↑
You've heard that Rails promotes “convention over configuration”.
Default rendering is an excellent example of this. By default, controllers
in Rails automatically render views with names that correspond to valid
routes. For example, if you have this code in your
BooksController
class:
class BooksController < ApplicationController end
And the following in your routes file:
resources :books
And you have a view file app/views/books/index.html.erb
:
<h1>Books are coming soon!</h1>
Rails will automatically render app/views/books/index.html.erb
when you navigate to /books
and you will see “Books are coming
soon!” on your screen.
However a coming soon screen is only minimally useful, so you will soon
create your Book
model and add the index action to
BooksController
:
class BooksController < ApplicationController def index @books = Book.all end end
Note that we don't have explicit render at the end of the index action
in accordance with “convention over configuration” principle. The rule is
that if you do not explicitly render something at the end of a controller
action, Rails will automatically look for the
action_name.html.erb
template in the controller's view
path and render it. So in this case, Rails will render the
app/views/books/index.html.erb
file.
If we want to display the properties of all the books in our view, we can do so with an ERB template like this:
<h1>Listing Books</h1> <table> <tr> <th>Title</th> <th>Summary</th> <th></th> <th></th> <th></th> </tr> <% @books.each do |book| %> <tr> <td><%= book.title %></td> <td><%= book.content %></td> <td><%= link_to "Show", book %></td> <td><%= link_to "Edit", edit_book_path(book) %></td> <td><%= link_to "Remove", book, method: :delete, data: { confirm: "Are you sure?" } %></td> </tr> <% end %> </table> <br> <%= link_to "New book", new_book_path %>
NOTE: The actual rendering is done by subclasses of
ActionView::TemplateHandlers
. This guide does not dig into
that process, but it's important to know that the file extension on
your view controls the choice of template handler. Beginning with Rails 2,
the standard extensions are .erb
for ERB (HTML with embedded
Ruby), and .builder
for Builder (XML generator).
Using render
¶ ↑
In most cases, the ActionController::Base#render
method does
the heavy lifting of rendering your application's content for use by a
browser. There are a variety of ways to customize the behavior of
render
. You can render the default view for a Rails template,
or a specific template, or a file, or inline code, or nothing at all. You
can render text, JSON, or XML. You can specify the content type or HTTP
status of the rendered response as well.
TIP: If you want to see the exact results of a call to render
without needing to inspect it in a browser, you can call
render_to_string
. This method takes exactly the same options
as render
, but it returns a string instead of sending a
response back to the browser.
Rendering Nothing¶ ↑
Perhaps the simplest thing you can do with render
is to render
nothing at all:
render nothing: true
If you look at the response for this using cURL, you will see the following:
$ curl -i 127.0.0.1:3000/books HTTP/1.1 200 OK Connection: close Date: Sun, 24 Jan 2010 09:25:18 GMT Transfer-Encoding: chunked Content-Type: */*; charset=utf-8 X-Runtime: 0.014297 Set-Cookie: _blog_session=...snip...; path=/; HttpOnly Cache-Control: no-cache $
We see there is an empty response (no data after the
Cache-Control
line), but the request was successful because
Rails has set the response to 200 OK. You can set the :status
option on render to change this response. Rendering nothing can be useful
for Ajax requests where all you want to send back to the browser is an
acknowledgment that the request was completed.
TIP: You should probably be using the head
method, discussed
later in this guide, instead of render :nothing
. This provides
additional flexibility and makes it explicit that you're only
generating HTTP headers.
Rendering an Action's View¶ ↑
If you want to render the view that corresponds to a different template
within the same controller, you can use render
with the name
of the view:
def update @book = Book.find(params[:id]) if @book.update(book_params) redirect_to(@book) else render "edit" end end
If the call to update
fails, calling the update
action in this controller will render the edit.html.erb
template belonging to the same controller.
If you prefer, you can use a symbol instead of a string to specify the action to render:
def update @book = Book.find(params[:id]) if @book.update(book_params) redirect_to(@book) else render :edit end end
Rendering an Action's Template from Another Controller¶ ↑
What if you want to render a template from an entirely different controller
from the one that contains the action code? You can also do that with
render
, which accepts the full path (relative to
app/views
) of the template to render. For example, if
you're running code in an AdminProductsController
that
lives in app/controllers/admin
, you can render the results of
an action to a template in app/views/products
this way:
render "products/show"
Rails knows that this view belongs to a different controller because of the
embedded slash character in the string. If you want to be explicit, you can
use the :template
option (which was required on Rails 2.2 and
earlier):
render template: "products/show"
Rendering an Arbitrary File¶ ↑
The render
method can also use a view that's entirely
outside of your application (perhaps you're sharing views between two
Rails applications):
render "/u/apps/warehouse_app/current/app/views/products/show"
Rails determines that this is a file render because of the leading slash
character. To be explicit, you can use the :file
option (which
was required on Rails 2.2 and earlier):
render file: "/u/apps/warehouse_app/current/app/views/products/show"
The :file
option takes an absolute file-system path. Of
course, you need to have rights to the view that you're using to render
the content.
NOTE: By default, the file is rendered using the current layout.
TIP: If you're running Rails on Microsoft Windows, you should use the
:file
option to render a file, because Windows filenames do
not have the same format as Unix filenames.
Wrapping it up¶ ↑
The above three ways of rendering (rendering another template within the controller, rendering a template within another controller and rendering an arbitrary file on the file system) are actually variants of the same action.
In fact, in the BooksController class, inside of the update action where we
want to render the edit template if the book does not update successfully,
all of the following render calls would all render the
edit.html.erb
template in the views/books
directory:
render :edit render action: :edit render "edit" render "edit.html.erb" render action: "edit" render action: "edit.html.erb" render "books/edit" render "books/edit.html.erb" render template: "books/edit" render template: "books/edit.html.erb" render "/path/to/rails/app/views/books/edit" render "/path/to/rails/app/views/books/edit.html.erb" render file: "/path/to/rails/app/views/books/edit" render file: "/path/to/rails/app/views/books/edit.html.erb"
Which one you use is really a matter of style and convention, but the rule of thumb is to use the simplest one that makes sense for the code you are writing.
Using render
with :inline
¶ ↑
The render
method can do without a view completely, if
you're willing to use the :inline
option to supply ERB as
part of the method call. This is perfectly valid:
render inline: "<% products.each do |p| %><p><%= p.name %></p><% end %>"
WARNING: There is seldom any good reason to use this option. Mixing ERB into your controllers defeats the MVC orientation of Rails and will make it harder for other developers to follow the logic of your project. Use a separate erb view instead.
By default, inline rendering uses ERB. You can force it to use Builder
instead with the :type
option:
render inline: "xml.p {'Horrid coding practice!'}", type: :builder
Rendering Text¶ ↑
You can send plain text - with no markup at all - back to the browser by
using the :plain
option to render
:
render plain: "OK"
TIP: Rendering pure text is most useful when you're responding to Ajax or web service requests that are expecting something other than proper HTML.
NOTE: By default, if you use the :plain
option, the text is
rendered without using the current layout. If you want Rails to put the
text into the current layout, you need to add the layout: true
option and use the .txt.erb
extension for the layout file.
Rendering HTML¶ ↑
You can send a HTML string back to the browser by using the
:html
option to render
:
render html: "<strong>Not Found</strong>".html_safe
TIP: This is useful when you're rendering a small snippet of HTML code. However, you might want to consider moving it to a template file if the markup is complex.
NOTE: This option will escape HTML entities if the string is not HTML safe.
Rendering JSON¶ ↑
JSON is a JavaScript data format used by many Ajax libraries. Rails has built-in support for converting objects to JSON and rendering that JSON back to the browser:
render json: @product
TIP: You don't need to call to_json
on the object that you
want to render. If you use the :json
option,
render
will automatically call to_json
for you.
Rendering XML¶ ↑
Rails also has built-in support for converting objects to XML and rendering that XML back to the caller:
render xml: @product
TIP: You don't need to call to_xml
on the object that you
want to render. If you use the :xml
option,
render
will automatically call to_xml
for you.
Rendering Vanilla JavaScript¶ ↑
Rails can render vanilla JavaScript:
render js: "alert('Hello Rails');"
This will send the supplied string to the browser with a MIME type of
text/javascript
.
Rendering raw body¶ ↑
You can send a raw content back to the browser, without setting any content
type, by using the :body
option to render
:
render body: "raw"
TIP: This option should be used only if you don't care about the
content type of the response. Using :plain
or
:html
might be more appropriate in most of the time.
NOTE: Unless overridden, your response returned from this render option
will be text/html
, as that is the default content type of
Action Dispatch response.
Options for render
¶ ↑
Calls to the render
method generally accept four options:
-
:content_type
-
:layout
-
:location
-
:status
The :content_type
Option¶ ↑
By default, Rails will serve the results of a rendering operation with the
MIME content-type of text/html
(or
application/json
if you use the :json
option, or
application/xml
for the :xml
option.). There are
times when you might like to change this, and you can do so by setting the
:content_type
option:
render file: filename, content_type: "application/rss"
The :layout
Option¶ ↑
With most of the options to render
, the rendered content is
displayed as part of the current layout. You'll learn more about
layouts and how to use them later in this guide.
You can use the :layout
option to tell Rails to use a specific
file as the layout for the current action:
render layout: "special_layout"
You can also tell Rails to render with no layout at all:
render layout: false
The :location
Option¶ ↑
You can use the :location
option to set the HTTP
Location
header:
render xml: photo, location: photo_url(photo)
The :status
Option¶ ↑
Rails will automatically generate a response with the correct HTTP status
code (in most cases, this is 200 OK
). You can use the
:status
option to change this:
render status: 500 render status: :forbidden
Rails understands both numeric status codes and the corresponding symbols shown below.
| Response Class | HTTP Status Code | Symbol | | ——————- | —————- | ——————————– | | Informational | 100 | :continue | | | 101 | :switching_protocols | | | 102 | :processing | | Success | 200 | :ok | | | 201 | :created | | | 202 | :accepted | | | 203 | :non_authoritative_information | | | 204 | :no_content | | | 205 | :reset_content | | | 206 | :partial_content | | | 207 | :multi_status | | | 208 | :already_reported | | | 226 | :im_used | | Redirection | 300 | :multiple_choices | | | 301 | :moved_permanently | | | 302 | :found | | | 303 | :see_other | | | 304 | :not_modified | | | 305 | :use_proxy | | | 306 | :reserved | | | 307 | :temporary_redirect | | | 308 | :permanent_redirect | | Client Error | 400 | :bad_request | | | 401 | :unauthorized | | | 402 | :payment_required | | | 403 | :forbidden | | | 404 | :not_found | | | 405 | :method_not_allowed | | | 406 | :not_acceptable | | | 407 | :proxy_authentication_required | | | 408 | :request_timeout | | | 409 | :conflict | | | 410 | :gone | | | 411 | :length_required | | | 412 | :precondition_failed | | | 413 | :request_entity_too_large | | | 414 | :request_uri_too_long | | | 415 | :unsupported_media_type | | | 416 | :requested_range_not_satisfiable | | | 417 | :expectation_failed | | | 422 | :unprocessable_entity | | | 423 | :locked | | | 424 | :failed_dependency | | | 426 | :upgrade_required | | | 428 | :precondition_required | | | 429 | :too_many_requests | | | 431 | :request_header_fields_too_large | | Server Error | 500 | :internal_server_error | | | 501 | :not_implemented | | | 502 | :bad_gateway | | | 503 | :service_unavailable | | | 504 | :gateway_timeout | | | 505 | :http_version_not_supported | | | 506 | :variant_also_negotiates | | | 507 | :insufficient_storage | | | 508 | :loop_detected | | | 510 | :not_extended | | | 511 | :network_authentication_required |
Finding Layouts¶ ↑
To find the current layout, Rails first looks for a file in
app/views/layouts
with the same base name as the controller.
For example, rendering actions from the PhotosController
class
will use app/views/layouts/photos.html.erb
(or
app/views/layouts/photos.builder
). If there is no such
controller-specific layout, Rails will use
app/views/layouts/application.html.erb
or
app/views/layouts/application.builder
. If there is no
.erb
layout, Rails will use a .builder
layout if
one exists. Rails also provides several ways to more precisely assign
specific layouts to individual controllers and actions.
Specifying Layouts for Controllers¶ ↑
You can override the default layout conventions in your controllers by
using the layout
declaration. For example:
class ProductsController < ApplicationController layout "inventory" #... end
With this declaration, all of the views rendered by the
ProductsController
will use
app/views/layouts/inventory.html.erb
as their layout.
To assign a specific layout for the entire application, use a
layout
declaration in your ApplicationController
class:
class ApplicationController < ActionController::Base layout "main" #... end
With this declaration, all of the views in the entire application will use
app/views/layouts/main.html.erb
for their layout.
Choosing Layouts at Runtime¶ ↑
You can use a symbol to defer the choice of layout until a request is processed:
class ProductsController < ApplicationController layout :products_layout def show @product = Product.find(params[:id]) end private def products_layout @current_user.special? ? "special" : "products" end end
Now, if the current user is a special user, they'll get a special layout when viewing a product.
You can even use an inline method, such as a Proc, to determine the layout.
For example, if you pass a Proc object, the block you give the Proc will be
given the controller
instance, so the layout can be determined
based on the current request:
class ProductsController < ApplicationController layout Proc.new { |controller| controller.request.xhr? ? "popup" : "application" } end
Conditional Layouts¶ ↑
Layouts specified at the controller level support the :only
and :except
options. These options take either a method name,
or an array of method names, corresponding to method names within the
controller:
class ProductsController < ApplicationController layout "product", except: [:index, :rss] end
With this declaration, the product
layout would be used for
everything but the rss
and index
methods.
Layout Inheritance¶ ↑
Layout declarations cascade downward in the hierarchy, and more specific layout declarations always override more general ones. For example:
-
application_controller.rb
class ApplicationController < ActionController::Base layout "main" end
-
articles_controller.rb
class ArticlesController < ApplicationController end
-
special_articles_controller.rb
class SpecialArticlesController < ArticlesController layout "special" end
-
old_articles_controller.rb
class OldArticlesController < SpecialArticlesController layout false def show @article = Article.find(params[:id]) end def index @old_articles = Article.older render layout: "old" end # ... end
In this application:
-
In general, views will be rendered in the
main
layout -
ArticlesController#index
will use themain
layout -
SpecialArticlesController#index
will use thespecial
layout -
OldArticlesController#show
will use no layout at all -
OldArticlesController#index
will use theold
layout
Avoiding Double Render Errors¶ ↑
Sooner or later, most Rails developers will see the error message “Can only
render or redirect once per action”. While this is annoying, it's
relatively easy to fix. Usually it happens because of a fundamental
misunderstanding of the way that render
works.
For example, here's some code that will trigger this error:
def show @book = Book.find(params[:id]) if @book.special? render action: "special_show" end render action: "regular_show" end
If @book.special?
evaluates to true
, Rails will
start the rendering process to dump the @book
variable into
the special_show
view. But this will not stop the
rest of the code in the show
action from running, and when
Rails hits the end of the action, it will start to render the
regular_show
view - and throw an error. The solution is
simple: make sure that you have only one call to render
or
redirect
in a single code path. One thing that can help is
and return
. Here's a patched version of the method:
def show @book = Book.find(params[:id]) if @book.special? render action: "special_show" and return end render action: "regular_show" end
Make sure to use and return
instead of &&
return
because && return
will not work due to
the operator precedence in the Ruby Language.
Note that the implicit render done by ActionController detects if
render
has been called, so the following will work without
errors:
def show @book = Book.find(params[:id]) if @book.special? render action: "special_show" end end
This will render a book with special?
set with the
special_show
template, while other books will render with the
default show
template.
Using redirect_to
¶ ↑
Another way to handle returning responses to an HTTP request is with
redirect_to
. As you've seen, render
tells
Rails which view (or other asset) to use in constructing a response. The
redirect_to
method does something completely different: it
tells the browser to send a new request for a different URL. For example,
you could redirect from wherever you are in your code to the index of
photos in your application with this call:
redirect_to photos_url
You can use redirect_to
with any arguments that you could use
with link_to
or url_for
. There's also a
special redirect that sends the user back to the page they just came from:
redirect_to :back
Getting a Different Redirect Status Code¶ ↑
Rails uses HTTP status code 302, a temporary redirect, when you call
redirect_to
. If you'd like to use a different status code,
perhaps 301, a permanent redirect, you can use the :status
option:
redirect_to photos_path, status: 301
Just like the :status
option for render
,
:status
for redirect_to
accepts both numeric and
symbolic header designations.
The Difference Between render
and redirect_to
¶ ↑
Sometimes inexperienced developers think of redirect_to
as a
sort of goto
command, moving execution from one place to
another in your Rails code. This is not correct. Your code stops
running and waits for a new request for the browser. It just happens that
you've told the browser what request it should make next, by sending
back an HTTP 302 status code.
Consider these actions to see the difference:
def index @books = Book.all end def show @book = Book.find_by(id: params[:id]) if @book.nil? render action: "index" end end
With the code in this form, there will likely be a problem if the
@book
variable is nil
. Remember, a render
:action
doesn't run any code in the target action, so nothing
will set up the @books
variable that the index
view will probably require. One way to fix this is to redirect instead of
rendering:
def index @books = Book.all end def show @book = Book.find_by(id: params[:id]) if @book.nil? redirect_to action: :index end end
With this code, the browser will make a fresh request for the index page,
the code in the index
method will run, and all will be well.
The only downside to this code is that it requires a round trip to the
browser: the browser requested the show action with /books/1
and the controller finds that there are no books, so the controller sends
out a 302 redirect response to the browser telling it to go to
/books/
, the browser complies and sends a new request back to
the controller asking now for the index
action, the controller
then gets all the books in the database and renders the index template,
sending it back down to the browser which then shows it on your screen.
While in a small application, this added latency might not be a problem, it is something to think about if response time is a concern. We can demonstrate one way to handle this with a contrived example:
def index @books = Book.all end def show @book = Book.find_by(id: params[:id]) if @book.nil? @books = Book.all flash.now[:alert] = "Your book was not found" render "index" end end
This would detect that there are no books with the specified ID, populate
the @books
instance variable with all the books in the model,
and then directly render the index.html.erb
template,
returning it to the browser with a flash alert message to tell the user
what happened.
Using head
To Build Header-Only Responses¶ ↑
The head
method can be used to send responses with only
headers to the browser. It provides a more obvious alternative to calling
render :nothing
. The head
method accepts a number
or symbol (see reference table)
representing a HTTP status code. The options argument is interpreted as a
hash of header names and values. For example, you can return only an error
header:
head :bad_request
This would produce the following header:
HTTP/1.1 400 Bad Request Connection: close Date: Sun, 24 Jan 2010 12:15:53 GMT Transfer-Encoding: chunked Content-Type: text/html; charset=utf-8 X-Runtime: 0.013483 Set-Cookie: _blog_session=...snip...; path=/; HttpOnly Cache-Control: no-cache
Or you can use other HTTP headers to convey other information:
head :created, location: photo_path(@photo)
Which would produce:
HTTP/1.1 201 Created Connection: close Date: Sun, 24 Jan 2010 12:16:44 GMT Transfer-Encoding: chunked Location: /photos/1 Content-Type: text/html; charset=utf-8 X-Runtime: 0.083496 Set-Cookie: _blog_session=...snip...; path=/; HttpOnly Cache-Control: no-cache
Structuring Layouts¶ ↑
When Rails renders a view as a response, it does so by combining the view with the current layout, using the rules for finding the current layout that were covered earlier in this guide. Within a layout, you have access to three tools for combining different bits of output to form the overall response:
-
Asset tags
-
yield
andcontent_for
-
Partials
Asset Tag Helpers¶ ↑
Asset tag helpers provide methods for generating HTML that link views to feeds, JavaScript, stylesheets, images, videos and audios. There are six asset tag helpers available in Rails:
-
auto_discovery_link_tag
-
javascript_include_tag
-
stylesheet_link_tag
-
image_tag
-
video_tag
-
audio_tag
You can use these tags in layouts or other views, although the
auto_discovery_link_tag
, javascript_include_tag
,
and stylesheet_link_tag
, are most commonly used in the
<head>
section of a layout.
WARNING: The asset tag helpers do not verify the existence of the assets at the specified locations; they simply assume that you know what you're doing and generate the link.
Linking to Feeds with the auto_discovery_link_tag
¶ ↑
The auto_discovery_link_tag
helper builds HTML that most
browsers and feed readers can use to detect the presence of RSS or Atom
feeds. It takes the type of the link (:rss
or
:atom
), a hash of options that are passed through to url_for,
and a hash of options for the tag:
<%= auto_discovery_link_tag(:rss, {action: "feed"}, {title: "RSS Feed"}) %>
There are three tag options available for the
auto_discovery_link_tag
:
-
:rel
specifies therel
value in the link. The default value is “alternate”. -
:type
specifies an explicit MIME type. Rails will generate an appropriate MIME type automatically. -
:title
specifies the title of the link. The default value is the uppercase:type
value, for example, “ATOM” or “RSS”.
Linking to JavaScript Files with the javascript_include_tag
¶ ↑
The javascript_include_tag
helper returns an HTML
script
tag for each source provided.
If you are using Rails with the Asset
Pipeline enabled, this helper will generate a link to
/assets/javascripts/
rather than
public/javascripts
which was used in earlier versions of
Rails. This link is then served by the asset pipeline.
A JavaScript file within a Rails application or Rails engine goes in one of
three locations: app/assets
, lib/assets
or
vendor/assets
. These locations are explained in detail in the
Asset Organization section
in the Asset Pipeline Guide
You can specify a full path relative to the document root, or a URL, if you
prefer. For example, to link to a JavaScript file that is inside a
directory called javascripts
inside of one of
app/assets
, lib/assets
or
vendor/assets
, you would do this:
<%= javascript_include_tag "main" %>
Rails will then output a script
tag such as this:
<script src='/assets/main.js'></script>
The request to this asset is then served by the Sprockets gem.
To include multiple files such as
app/assets/javascripts/main.js
and
app/assets/javascripts/columns.js
at the same time:
<%= javascript_include_tag "main", "columns" %>
To include app/assets/javascripts/main.js
and
app/assets/javascripts/photos/columns.js
:
<%= javascript_include_tag "main", "/photos/columns" %>
To include http://example.com/main.js
:
<%= javascript_include_tag "http://example.com/main.js" %>
Linking to CSS Files with the stylesheet_link_tag
¶ ↑
The stylesheet_link_tag
helper returns an HTML
<link>
tag for each source provided.
If you are using Rails with the “Asset Pipeline” enabled, this helper will
generate a link to /assets/stylesheets/
. This link is then
processed by the Sprockets gem. A stylesheet file can be stored in one of
three locations: app/assets
, lib/assets
or
vendor/assets
.
You can specify a full path relative to the document root, or a URL. For
example, to link to a stylesheet file that is inside a directory called
stylesheets
inside of one of app/assets
,
lib/assets
or vendor/assets
, you would do this:
<%= stylesheet_link_tag "main" %>
To include app/assets/stylesheets/main.css
and
app/assets/stylesheets/columns.css
:
<%= stylesheet_link_tag "main", "columns" %>
To include app/assets/stylesheets/main.css
and
app/assets/stylesheets/photos/columns.css
:
<%= stylesheet_link_tag "main", "photos/columns" %>
To include http://example.com/main.css
:
<%= stylesheet_link_tag "http://example.com/main.css" %>
By default, the stylesheet_link_tag
creates links with
media="screen" rel="stylesheet"
. You can
override any of these defaults by specifying an appropriate option
(:media
, :rel
):
<%= stylesheet_link_tag "main_print", media: "print" %>
Linking to Images with the image_tag
¶ ↑
The image_tag
helper builds an HTML <img />
tag to the specified file. By default, files are loaded from
public/images
.
WARNING: Note that you must specify the extension of the image.
<%= image_tag "header.png" %>
You can supply a path to the image if you like:
<%= image_tag "icons/delete.gif" %>
You can supply a hash of additional HTML options:
<%= image_tag "icons/delete.gif", {height: 45} %>
You can supply alternate text for the image which will be used if the user has images turned off in their browser. If you do not specify an alt text explicitly, it defaults to the file name of the file, capitalized and with no extension. For example, these two image tags would return the same code:
<%= image_tag "home.gif" %> <%= image_tag "home.gif", alt: "Home" %>
You can also specify a special size tag, in the format “{width}x{height}”:
<%= image_tag "home.gif", size: "50x20" %>
In addition to the above special tags, you can supply a final hash of
standard HTML options, such as :class
, :id
or
:name
:
<%= image_tag "home.gif", alt: "Go Home", id: "HomeImage", class: "nav_bar" %>
Linking to Videos with the video_tag
¶ ↑
The video_tag
helper builds an HTML 5
<video>
tag to the specified file. By default, files are
loaded from public/videos
.
<%= video_tag "movie.ogg" %>
Produces
<video src="/videos/movie.ogg" />
Like an image_tag
you can supply a path, either absolute, or
relative to the public/videos
directory. Additionally you can
specify the size: "#{width}x#{height}"
option just
like an image_tag
. Video tags can also have any of the HTML
options specified at the end (id
, class
et al).
The video tag also supports all of the <video>
HTML
options through the HTML options hash, including:
-
poster: "image_name.png"
, provides an image to put in place of the video before it starts playing. -
autoplay: true
, starts playing the video on page load. -
loop: true
, loops the video once it gets to the end. -
controls: true
, provides browser supplied controls for the user to interact with the video. -
autobuffer: true
, the video will pre load the file for the user on page load.
You can also specify multiple videos to play by passing an array of videos
to the video_tag
:
<%= video_tag ["trailer.ogg", "movie.ogg"] %>
This will produce:
<video><source src="/videos/trailer.ogg" /><source src="/videos/trailer.flv" /></video>
Linking to Audio Files with the audio_tag
¶ ↑
The audio_tag
helper builds an HTML 5
<audio>
tag to the specified file. By default, files are
loaded from public/audios
.
<%= audio_tag "music.mp3" %>
You can supply a path to the audio file if you like:
<%= audio_tag "music/first_song.mp3" %>
You can also supply a hash of additional options, such as :id
,
:class
etc.
Like the video_tag
, the audio_tag
has special
options:
-
autoplay: true
, starts playing the audio on page load -
controls: true
, provides browser supplied controls for the user to interact with the audio. -
autobuffer: true
, the audio will pre load the file for the user on page load.
Understanding yield
¶ ↑
Within the context of a layout, yield
identifies a section
where content from the view should be inserted. The simplest way to use
this is to have a single yield
, into which the entire contents
of the view currently being rendered is inserted:
<html> <head> </head> <body> <%= yield %> </body> </html>
You can also create a layout with multiple yielding regions:
<html> <head> <%= yield :head %> </head> <body> <%= yield %> </body> </html>
The main body of the view will always render into the unnamed
yield
. To render content into a named yield
, you
use the content_for
method.
Using the content_for
Method¶ ↑
The content_for
method allows you to insert content into a
named yield
block in your layout. For example, this view would
work with the layout that you just saw:
<% content_for :head do %> <title>A simple page</title> <% end %> <p>Hello, Rails!</p>
The result of rendering this page into the supplied layout would be this HTML:
<html> <head> <title>A simple page</title> </head> <body> <p>Hello, Rails!</p> </body> </html>
The content_for
method is very helpful when your layout
contains distinct regions such as sidebars and footers that should get
their own blocks of content inserted. It's also useful for inserting
tags that load page-specific JavaScript or css files into the header of an
otherwise generic layout.
Using Partials¶ ↑
Partial templates - usually just called “partials” - are another device for breaking the rendering process into more manageable chunks. With a partial, you can move the code for rendering a particular piece of a response to its own file.
Naming Partials¶ ↑
To render a partial as part of a view, you use the render
method within the view:
<%Q= render "menu" %>
This will render a file named _menu.html.erb
at that point
within the view being rendered. Note the leading underscore character:
partials are named with a leading underscore to distinguish them from
regular views, even though they are referred to without the underscore.
This holds true even when you're pulling in a partial from another
folder:
<%Q= render "shared/menu" %>
That code will pull in the partial from
app/views/shared/_menu.html.erb
.
Using Partials to Simplify Views¶ ↑
One way to use partials is to treat them as the equivalent of subroutines: as a way to move details out of a view so that you can grasp what's going on more easily. For example, you might have a view that looked like this:
<%= render "shared/ad_banner" %> <h1>Products</h1> <p>Here are a few of our fine products:</p> ... <%= render "shared/footer" %>
Here, the _ad_banner.html.erb
and
_footer.html.erb
partials could contain content that is shared
among many pages in your application. You don't need to see the details
of these sections when you're concentrating on a particular page.
TIP: For content that is shared among all pages in your application, you can use partials directly from layouts.
Partial Layouts¶ ↑
A partial can use its own layout file, just as a view can use a layout. For example, you might call a partial like this:
<%= render partial: "link_area", layout: "graybar" %>
This would look for a partial named _link_area.html.erb
and
render it using the layout _graybar.html.erb
. Note that
layouts for partials follow the same leading-underscore naming as regular
partials, and are placed in the same folder with the partial that they
belong to (not in the master layouts
folder).
Also note that explicitly specifying :partial
is required when
passing additional options such as :layout
.
Passing Local Variables¶ ↑
You can also pass local variables into partials, making them even more powerful and flexible. For example, you can use this technique to reduce duplication between new and edit pages, while still keeping a bit of distinct content:
-
new.html.erb
<h1>New zone</h1> <%= render partial: "form", locals: {zone: @zone} %>
-
edit.html.erb
<h1>Editing zone</h1> <%= render partial: "form", locals: {zone: @zone} %>
-
_form.html.erb
<%= form_for(zone) do |f| %> <p> <b>Zone name</b><br> <%= f.text_field :name %> </p> <p> <%= f.submit %> </p> <% end %>
Although the same partial will be rendered into both views, Action View's submit helper will return “Create Zone” for the new action and “Update Zone” for the edit action.
Every partial also has a local variable with the same name as the partial
(minus the underscore). You can pass an object in to this local variable
via the :object
option:
<%= render partial: "customer", object: @new_customer %>
Within the customer
partial, the customer
variable will refer to @new_customer
from the parent view.
If you have an instance of a model to render into a partial, you can use a shorthand syntax:
<%= render @customer %>
Assuming that the @customer
instance variable contains an
instance of the Customer
model, this will use
_customer.html.erb
to render it and will pass the local
variable customer
into the partial which will refer to the
@customer
instance variable in the parent view.
Rendering Collections¶ ↑
Partials are very useful in rendering collections. When you pass a
collection to a partial via the :collection
option, the
partial will be inserted once for each member in the collection:
-
index.html.erb
<h1>Products</h1> <%= render partial: "product", collection: @products %>
-
_product.html.erb
<p>Product Name: <%= product.name %></p>
When a partial is called with a pluralized collection, then the individual
instances of the partial have access to the member of the collection being
rendered via a variable named after the partial. In this case, the partial
is _product
, and within the _product
partial, you
can refer to product
to get the instance that is being
rendered.
There is also a shorthand for this. Assuming @products
is a
collection of product
instances, you can simply write this in
the index.html.erb
to produce the same result:
<h1>Products</h1> <%= render @products %>
Rails determines the name of the partial to use by looking at the model name in the collection. In fact, you can even create a heterogeneous collection and render it this way, and Rails will choose the proper partial for each member of the collection:
-
index.html.erb
<h1>Contacts</h1> <%= render [customer1, employee1, customer2, employee2] %>
-
customers/_customer.html.erb
<p>Customer: <%= customer.name %></p>
-
employees/_employee.html.erb
<p>Employee: <%= employee.name %></p>
In this case, Rails will use the customer or employee partials as appropriate for each member of the collection.
In the event that the collection is empty, render
will return
nil, so it should be fairly simple to provide alternative content.
<h1>Products</h1> <%= render(@products) || "There are no products available." %>
Local Variables¶ ↑
To use a custom local variable name within the partial, specify the
:as
option in the call to the partial:
<%= render partial: "product", collection: @products, as: :item %>
With this change, you can access an instance of the @products
collection as the item
local variable within the partial.
You can also pass in arbitrary local variables to any partial you are
rendering with the locals: {}
option:
<%= render partial: "product", collection: @products, as: :item, locals: {title: "Products Page"} %>
In this case, the partial will have access to a local variable
title
with the value “Products Page”.
TIP: Rails also makes a counter variable available within a partial called
by the collection, named after the member of the collection followed by
_counter
. For example, if you're rendering
@products
, within the partial you can refer to
product_counter
to tell you how many times the partial has
been rendered. This does not work in conjunction with the as:
:value
option.
You can also specify a second partial to be rendered between instances of
the main partial by using the :spacer_template
option:
Spacer Templates¶ ↑
<%= render partial: @products, spacer_template: "product_ruler" %>
Rails will render the _product_ruler
partial (with no data
passed in to it) between each pair of _product
partials.
Collection Partial Layouts¶ ↑
When rendering collections it is also possible to use the
:layout
option:
<%= render partial: "product", collection: @products, layout: "special_layout" %>
The layout will be rendered together with the partial for each item in the collection. The current object and object_counter variables will be available in the layout as well, the same way they do within the partial.
Using Nested Layouts¶ ↑
You may find that your application requires a layout that differs slightly from your regular application layout to support one particular controller. Rather than repeating the main layout and editing it, you can accomplish this by using nested layouts (sometimes called sub-templates). Here's an example:
Suppose you have the following ApplicationController
layout:
-
app/views/layouts/application.html.erb
<html> <head> <title><%= @page_title or "Page Title" %></title> <%= stylesheet_link_tag "layout" %> <style><%= yield :stylesheets %></style> </head> <body> <div id="top_menu">Top menu items here</div> <div id="menu">Menu items here</div> <div id="content"><%= content_for?(:content) ? yield(:content) : yield %></div> </body> </html>
On pages generated by NewsController
, you want to hide the top
menu and add a right menu:
-
app/views/layouts/news.html.erb
<% content_for :stylesheets do %> #top_menu {display: none} #right_menu {float: right; background-color: yellow; color: black} <% end %> <% content_for :content do %> <div id="right_menu">Right menu items here</div> <%= content_for?(:news_content) ? yield(:news_content) : yield %> <% end %> <%= render template: "layouts/application" %>
That's it. The News views will use the new layout, hiding the top menu and adding a new right menu inside the “content” div.
There are several ways of getting similar results with different
sub-templating schemes using this technique. Note that there is no limit in
nesting levels. One can use the ActionView::render
method via
render template: 'layouts/news'
to base a new layout
on the News layout. If you are sure you will not subtemplate the
News
layout, you can replace the
content_for?(:news_content) ? yield(:news_content) : yield
with simply yield
.